The present invention relates to a finned tube for the thermal cracking of hydrocarbons in the presence of steam, in which the charge mixture is passed through externally heated tubes with helical inner fins.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
Tube furnaces in which a hydrocarbon/steam mixture is passed through series of individual or meandering tubes (cracking tube coils) at temperatures of above 750° C. made from heat-resistant chromium-nickel-steel alloys with a high resistance to oxidation or scaling and a high resistance to carburization have proven suitable for the high-temperature pyrolysis of hydrocarbons (crude oil derivatives). The tube coils comprise vertically running, straight tube sections which are connected to one another via U-shaped tube bends or are arranged in parallel with one another; they are usually heated with the aid of side-wall burners and in some cases also with the aid of bottom burners and therefore have what is known as a light side, facing the burners, and what is known as a dark side, which is offset by 90° with respect thereto, i.e. runs in the direction of the rows of tubes. The mean tube metal temperatures (TMT) are in some cases over 1000° C.
The service life of the cracking tubes is dependent to a very significant extent on the creep resistance and the carburization resistance, and also the coking rate, of the tube material. A crucial factor for the coking rate, i.e. the growth of a layer of carbon deposits (pyrolysis coke) on the tube inner wall is, in addition to the type of hydrocarbons used, the cracking gas temperature in the region of the inner wall and what is known as the operating severity, which conceals the influence of the system pressure and the residence time in the tube system on the ethylene yield. The operating severity is set on the basis of the mean outlet temperature of the cracking gases (e.g. 850° C.). The higher the gas temperature in the vicinity of the tube inner wall above this temperature, the more extensive the growth of the layer of pyrolysis coke becomes, and the insulating action of this layer allows the tube metal temperature to increase still further. Although the chromium-nickel-steel alloys containing 0.4% of carbon, over 25% of chromium and over 20% of nickel, for example 35% of chromium, 45% of nickel and if appropriate 1% of niobium, that are used as tube material have a high resistance to carburization, the carbon diffuses into the tube wall at defects in the oxide layer, where it leads to considerable carburization which can amount to carbon contents of from 1% to 3% at wall depths of 0.5 to 3 mm. This is associated with considerable embrittlement of the tube material, with the risk of crack formation in the event of fluctuating thermal loads, in particular when the furnace is being started up and shut down.
To break down the carbon deposits (coking) on the tube inner wall, it is necessary for cracking operation to be interrupted from time to time and for the pyrolysis coke to be burnt with the aid of a steam/air mixture. This requires operation to be interrupted for up to 36 hours, and therefore has a considerable adverse effect on the economics of the process.
It is also known from GB Patent 969 796 to use cracking tubes with inner fins. Although inner fins of this type result in an internal surface area which is a good few percent, for example 10%, larger, with a corresponding improvement in the heat transfer, they are also associated with the drawback of a considerably increased pressure loss compared to a smooth tube, on account of friction at the enlarged tube inner surface. The higher pressure loss requires a higher system pressure, which inevitably changes the residence time and has an adverse effect on the yield. An additional factor is that the known tube materials with high carbon and chromium contents can no longer be profiled by cold-working, for example cold-drawing. They have the drawback that their deformability decreases greatly as the hot strength rises. This has led to the high tube metal temperatures of, for example, up to 1050° C., which are desirable with regard to the ethylene yield, requiring the use of centrifugally cast tubes. However, since centrifugally cast tubes can only be produced with a cylindrical wall, special shaping processes are required, for example removal of material by electrolytic machining or a shaping welding process if internally finned tubes are to be produced.
It would therefore be desirable and advantageous to improve the economics of thermal cracking of hydrocarbons in tubular furnaces with externally heated tubes having helical inner fins.